linux-kernel - Re: [PATCH 8/9] perf, x86: Optimize v4 LBR unfreezing

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Message-ID: <20150508111918.GE27504@twins.programming.kicks-ass.net>
Date:	Fri, 8 May 2015 13:19:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	kan.liang@...el.com, eranian@...gle.com, acme@...radead.org,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 8/9] perf, x86: Optimize v4 LBR unfreezing

On Thu, May 07, 2015 at 03:56:31PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> In Arch perfmon v4 the GLOBAL_STATUS reset automatically unfreezes
> LBRs. So no need to do it manually in the LBR code. Add a check
> to skip it.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_lbr.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_lbr.c b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> index 6c48c97..64d3122 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_lbr.c
> @@ -147,6 +147,13 @@ static void __intel_pmu_lbr_enable(bool pmi)
>  		wrmsrl(MSR_LBR_SELECT, lbr_select);
>  	}
>  
> +	/*
> +	 * No need to unfreeze manually, as v4 can do that as part
> +	 * of the GLOBAL_STATUS ack.
> +	 */
> +	if (pmi && x86_pmu.version >= 4)
> +		return;
> +

But that block above is !pmi, so strictly exclusive with this condition.
So why not put this at the start?

>  	rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
>  	orig_debugctl = debugctl;
>  	debugctl |= DEBUGCTLMSR_LBR;
--
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